1. Field of the Invention
The present invention relates to a method for manufacturing a ring-shaped magnet material, and manufacturing apparatus used therefor. More specifically, the invention relates to a method capable of manufacturing a ring-shaped magnet material excellent in magnetic properties continuously or by single taking, with high yield, and also capable of manufacturing with more freedom for design with regard to the required properties, and relates to a manufacturing apparatus used therefor.
2. Prior Art
In an Nd—Fe—B type fully dense permanent magnet, the one caused to have a magnetically radial anisotropic property by extrusion molding in particular is useful as the material for a ring-shaped magnet.
Conventionally, material for such a ring-shaped magnet has been manufactured as follows. First, for example, a melt spun magnetically isotropic ribbon made of a rare earth permanent magnet alloy is crushed into powder, which is cold pressed into a green compact. Then, this green compact is densified by warm-pressing or hot-pressing to thereby make a cylindrical preform with the desired dimensions, for example.
Then, for example, by carrying out backward-extrusion forming to this cylindrical preform in the warm, the crystal axis is orientation-disposed to exhibit a magnetic anisotropy property and at the same time a cup-shaped body having a desired geometry is once formed, and a piercing by means of a perforating punch is carried out to the portion corresponding to the bottom portion of this cup, thereby making an objective ring-shaped magnet material.
In addition, this ring-shaped magnet material is magnetized in the subsequent step, thereby being provided for practical use as the magnet having the radial anisotropy property.
However, because the above-described manufacturing method is a batch system, the productivity thereof is essentially low. Moreover, because the backward-extrusion is applied, a sufficient processing distortion is not applied to the preform in the initial stage of forming, and a tip portion formed in the initial stage will deteriorate in the magnetic properties as compared with the other portions. Therefore, for commercialization of the product, the portion concerned needs to be cut.
Namely, as a loss due to the punching of the bottom portion is also added, the yield of the product becomes extremely low in the above-described manufacturing method.
In order to solve such problems, in Japanese Unexamined Patent Publication No. Hei 9-129463, a method for manufacturing magnet materials is proposed as follows.
In this method, the ring-shaped magnet material is manufactured as follows. As shown in FIG. 1, in a penetrating hole 11A of a die 11 in which the penetrating hole 11A having a constant diameter is formed, a cylindrical mandrel 12 whose tip portion is a flat surface 12a and whose diameter is smaller than that of the penetrating hole 11A is arranged. On the top of this mandrel a preform made of magnetic powder is loaded and this preform is pressed with a pressing punch 13. The preform is pressed into a gap formed between the mandrel 12 and the die 11 to be plastic-deformed. Then, as shown in FIG. 1, at the time when the preform is extruded into a cup-shaped body 14′, the pressing punch 13 is pulled up, and a new preform of magnetic powder is loaded on the top of the cup-shaped body and presses with the pressing punch 13 again. During the process in which the newly loaded preform is plastic-deformed to be extruded into a new cup-shaped body 14′, the upper end of the cup-shaped body in the preceding stage is stuck to the lower end of the newly extruded cup-shaped body 14′ and is protruded downward in the penetrating hole 11A while being ring-shaped in the state of being coupled with the new cup-shaped body 14′.
Accordingly, in the case of this manufacturing method, by repeating the above-described operations sequentially, the ring-shaped magnet material 14 is extruded continuously and the productivity thereof is high. In addition, punching the bottom portion, cutting the tip portion and the like, which have been carried out with regard to each magnet material as in the case of the batch system, will not be required and the yield becomes high accordingly.
However, the continuous extruding method of the prior art described above has the following problems.
A first problem is that the coupling portion between the ring-shaped extrusion 14 positioned down below and the new cup-shaped body 14′ positioned up above is formed as shown in FIG. 1.
Namely, in the coupling portion the material of the ring-shaped extrusion 14 wraps around from inside to outside along the mandrel 12, and the material of the new cup-shaped body 14′ also wraps around from outside to inside along the die 11, and thus the coupling portion will not be a flat end face in which the upper end face of the ring-shaped extrusion 14 and the lower end face of the cup-shaped body 14′ intersect at right angles with the longitudinal direction.
For this reason, this part of the coupling portion needs to be cut from the continuous extrusion obtained, and consequently, an advantage that cutting the bottom portion is not required to thereby improve the yield of the product in the batch system will be canceled out.
A second problem is that the freedom for design with regard to the required magnetic properties is extremely narrow.
Generally, if the preform of magnetic material, which is the original material, is processed with a large reduction in area (amount of working), the magnetic properties of the ring-shaped magnet material obtained will be also improved.
However, in case of using this apparatus, if the specification (the outer diameter and inner diameter) for the target product is determined, the diameter of the penetrating hole of the die and the diameter of the mandrel will be determined uniquely. Accordingly, the reduction in area is also determined uniquely. Therefore, if the target geometry is determined, in the first place it is impossible to design the improvement of the magnetic properties by increasing the reduction in area with respect to the original material.
A third problem is that the ring-shaped magnet material manufactured will likely cause a core misalignment.
This is because the mandrel to be arranged in the penetrating hole of the die is relatively long and is used only in the state where the basic end portion thereof is one-point supported with mandrel backup means (not shown). Namely, because the mandrel is in the one-point supported state, the tip portion of the mandrel 12 may oscillate subtlety during the process of loading the preform into the tip portion of the mandrel, of subsequently pressing with the pressing punch 13, or the like. As a result, the core misalignment occurs, thereby deteriorating the dimension accuracy of the product.
A fourth problem is the problem that the magnetic properties of the ring-shaped magnet manufactured are not necessarily high. The demand for the miniaturization and advanced features in the recent electrical and electric equipments has become extremely strong, and in conjunction with this, for example, the magnetic properties on the order of: (BH) max of 400 kJ/m3; Br of 1.45 T; and iHc of 1220 kA/m are required for the ring-shaped magnet to be built into these equipments.
However, in the above-described method of the prior art, it is difficult to manufacture the ring-shaped magnet having such high magnetic properties. For this reason, a new method for manufacturing the ring-shaped magnet capable of enhancing the magnetic properties further is asked for.